To form a heated window of the type of a rear window of an automobile, a series of narrow resistance strips that are made from a pasty suspension in an organic binder of metallic silver and molten glass, in other words, a low-melting-point glass, are deposited on a glass film by screen printing. After baking, a network of fine electrically conductive strips is obtained that flow into larger collecting strips of analogous composition that are arranged near the edges of the glass film so as not to obstruct the driver's view. Metallic connecting parts that form cable shoes for the power supply of the vehicle are then soldered to these lateral collecting strips.
These metallic connecting parts are formed, for example, in a T shape, from stiff plates made of copper, whose two branches are extended by feet, under which the solder is deposited. The connecting part is heated by any known process, such as, for instance, by conduction or convection, to cause the solder to melt and to ensure, after cooling, a permanent connection between the collecting strip and the connecting part.
For a long time, the solder used has been lead based because of its suitable ductility. In fact, this metal enables minimization of the stresses that can occur the window glass in the solder zone during rapid cooling processes and increases in the temperature of the window pane that is provided with its stiff connecting part, for example, during heat resistance tests, for which the window panes are subjected, in a closed housing, to temperature fluctuation cycles between −40° C. and +90° C. inside this housing.
For environmental protection reasons, it is necessary to replace the lead with other materials, such as tin-based alloys, for instance. However, tin is a much less ductile material. Its presence between a stiff metallic connecting part and the glass film that is provided with components for the connection, does, in fact, ensure an electrical connection and mechanical retention of the part, but ultimately results in direct contact between the stiff part and the glass film. At the time of retraction of the metallic part, in particular, when it is subjected to a sharp cooling in temperature, this direct contact between the part and the glass causes, in principle, the occurrence of stresses in the glass. In connection with the fact that the glass used is becoming increasing thinner and that it cannot be annealed (i.e., is less robust), the stresses in the glass usually result in breakage of the window pane.
Another solution of the connecting element can consist in using a flat substrate made of plastic that is affixed to the glass film by gluing, with metallic elements embedded in this substrate and electrical connection points included on a free end, which connection points protrude from the substrate in order to be solidly connected with the electrical components of the window pane, with the electrical connection occurring with a stiff part of a power connector on the opposite end of the metallic elements and of the substrate made of plastic material. The plastic substrate permits affixing the connecting element and does not result in direct contact between the stiff part and the electrical components of the window pane.
Such a configuration is, for example, described in the patent EP 766 338 B1. However, this solution has proven to be expensive, in particular because of the necessity of the plastic substrate and of the encasing of the metallic elements inside this substrate.